Inflammatory bowel disease (IBD) is a complex inflammatory gastrointestinal (GI) disease involving multiple environmental and genetic risk factors. There are two main entities: Crohn's disease (CD) and ulcerative colitis (UC). Originally being considered as a disease of industrialized countries, IBD is now an emerging global disease as its prevalence is on the rise, in both children and adults, even in developing countries. There remains regional difference in its prevalence; with Europe and North America having the highest prevalence rates. Europe has the highest annual incidence of UC (24.3 per 100,000 person-years) and North American has the highest annual incidence of CD (20.2 per 100,000 person-years). In Europe and North American, there are approximately 3.6 million IBD patients suffering from this devastating chronic disease. In 2012, there are about 233,000 Canadians with IBD, and the estimated cost (including medical costs, indirect costs and long-term work losses) is about $2.8 billion. IBD patients suffer significant abdominal pain, recurrent diarrhea and some have severe complications such as colon cancer, sclerosing cholangitis, abscess, fistula formation and pouchitis. Thus, these patients encounter numerous challenges including poor quality of life, expensive medications, employment problems and lack of appropriate support. IBD patients have higher mortality rates compared to the general population.
Ankylosing spondylitis (AS) is a chronic and disabling spinal disease affecting predominantly young men. It is also a complex disease involving multiple risk factors. Genetics, environmental and other factors such as epi-genetics play a role in increasing the odds to developing the AS disease. The hallmark of AS is inflammation and ankylosis (fusion of joints) mainly at the cartilage/bone interface including the enthesis (where ligaments or tendons attach to bone). It can lead to significant spinal disease and peripheral arthritis. The current view considers new bone formation at the enthesis as a pathological response to injury and that joint inflammation precedes ossification. Early stages of ankylosis involve squaring of the vertebral bodies and formation of syndesmophytes. Total spinal ankylosis (“Bamboo spine”) is commonly found in the most severe cases. The cause of chronic joint inflammation is not known. There is an ongoing debate on whether chronic inflammation and new bone formation are linked events or independent processes. In North America, the prevalence of AS is about 1 in 200. Currently, there are about 200,000 Canadians with AS.
AS is usually diagnosed according to the modified New York criteria which include a combination of clinical features such as limited motion of the lumbar spine, persistent lower back pain, limited chest expansion; and radiographic evidence of sacroiliitis. Although joint inflammation can be detected early in the disease process (at least within the first year of symptom onset) using magnetic resonance imaging (MRI) technology, it is not a definitive diagnosis test for AS. The subsequent spinal structural changes as visualized on radiographs appear relatively late. This likely explains why it can take 5-10 years to confirm a diagnosis of AS after the initial onset of disease symptoms. Currently, monitoring of patient outcomes in AS relies on the detection of radiographic changes which represent irreversible structural damage. Radiographic changes of the spine (cervical and lumbar) in AS patients are scored using the modified Stoke AS Spine Score (mSASSS).
In summary, severe cases of both chronic diseases (IBD and AS) are extremely disabling and require life-time medical treatments. The availability of biomarkers could not only facilitate rapid diagnosis, but could also provide better assessment and improved ability to determine prognosis. 15-40% of IBD patients have extra-intestinal manifestations such as uveitis, primary sclerosing cholangitis and axial arthritis. Articular manifestations are the most common, affecting about 30% of IBD patients. Isolated sacroiliitis was detected on pelvic X-rays of 2-18% and on CT scans of 14-33% IBD patients respectively. Conversely, one of the extra-articular manifestations of ankylosing spondylitis (AS) is IBD, with 5-10% of AS patients having concomitant IBD. About 60% of AS patients have microscopic colitis identified by colonoscopy. Symptoms of AS can precede IBD symptoms in 31-50% of patients. In 15-40% of cases, both IBD and AS symptoms can occur simultaneously. In patients with concomitant IBD and AS, 39% had IBD symptoms before having AS symptoms, 52% had AS symptoms first and 9% had concurrent IBD and AS symptoms.
Biologic response modifiers (such as Tumor Necrosis Factor inhibitors [TNFi]) are effective in both CD and AS, implicating both diseases share common pathological pathways. However, not all CD/AS patients respond to this treatment. In CD, independent predictors of good response include shorter disease duration, younger age, high CRP, and isolated colitis, while isolated ileitis, strictures and smoking are associated with poor response to TNFi. In AS, shorter disease duration is a strong predictor of treatment response. Other clinical parameters associated with better response to TNFi in AS patients includes younger age, increased acute phase proteins, better functional status, male gender, and significant inflammation detected by MRI. We reported recently that in AS patient treated with TNFi, a combination of biomarkers could better predict treatment response than using acute phase proteins alone. Yet none of these parameters, even when used in combinations, have sufficient sensitivity and specificity to provide a personalized and optimal management of the disease. It is clear that a better understanding of the pathogenesis of these diseases is essential to facilitate accurate predictions of responders and identification of patients likely to develop adverse effects. Treatments with biologics (such as TNFi) are expensive and associated with a significant side effect profile including increased risk of infections and malignancies. Hence, it is imperative to identify biomarkers to screen patients who are likely to benefit from this treatment and in addition to monitor their response to treatment in a timely manner in order to avoid potential serious adverse effects.
Genetics plays a key role in both IBD and AS. While only 8% of Caucasians are HLA-B27 positive, both primary AS and IBD-related AS are associated with B27 positivity. The association is much stronger in AS patients (more than 90%) than in patients with both AS and IBD (25-78%). Recent genetic analyses on complex diseases using genome-wide association (GWA) studies have been extremely successful in revealing IBD susceptibility loci, in particular those that are shared with other inflammatory diseases such as AS. The most recent estimate includes 163 IBD loci, and about 28 of them were shared between CD and UC. The recent immunochip study identified 12 and 11 AS loci shared with CD and UC respectively. A major achievement of these genetic studies relates to the identification of important biological pathways likely lead to shared pathogenesis in IBD and AS. The most notable non-MHC susceptibility loci shared between CD and AS are ERAP2 and IL23R, implicating common pathways in antigen presentation and adaptive immunity respectively.
AS and IBD patients share similar intestinal inflammation based on histopathological findings. Recent ileocolonoscopy studies suggested that AS and CD patients shared indistinguishable gut histopathology even in the absence of GI symptoms in some AS patients. The importance of IL23 pathway as implicated by genetic studies is reiterated by the finding that comparable IL23 production was detected in the guts of CD and AS patients. The link between gut and joint inflammation suggests a common etiology between IBD and AS, but its origin and underlying pathophysiology remain largely unclear. Better understanding of the pathological basis for the coexistence of AS and IBD would impact on improved management of these patients.
Despite these recent advances, there are currently no biomarkers that can predict the occurrence of ankylosis in IBD, or GI symptoms in AS.
Numerous studies have reported dysbioses/imbalance in the gut microbiome of IBD patients, and these dysbioses are associated with alterations in microbial metabolic functions which could impact on the gut of the host. In these patients, there is decreased bacterial biodiversity. Specific features of CD-associated dysbioses include altered proportion of the Clostridia bacteria, and a significant increase in Enterobacteria (specifically Escherichia/Shigella: bacteria implicated in gut inflammation). Treatments for IBD were associated with changes of gut microbiome, implicating gut microbiome likely causes IBD. For example, the use of a bowel-specific aminosalicylate drug (5-aminosalicylic acid) was linked to a significant reduction of Escherichia/Shigella. Aside from studies implicating that affected ileum in CD patients likely provides a favorable environment for the establishment of pathological E. coli, CD patients are susceptible to Clostridium difficile infections. Gut-resident microbial antigens are candidates for triggering and sustaining chronic inflammation, leading to changes in the host immune responses.
It is less clear whether AS patients have distinct gut microbiome. For decades, a relationship between reactive arthritis (ReA) and certain enterobacteria (e.g. Salmonella, Yersinia, Shigella and Klebsiella) has been noted. Some ReA patients eventually develop AS. The evidence for the association between AS and bacterial infections remains unresolved, though Klebsiella was found to colonize in the guts of AS patients.
In sera of CD and UC patients, numerous signature antibodies have been detected. These include anti-neutrophil cytoplasmic antibodies and antimicrobial antibodies. Representative antimicrobial antibodies are anti-Saccharomyces cerevisiae mannan (ASCAs) antibodies, antibodies recognizing Escherichia coli outer membrane porin C (OmpC) and the flagellin CBir1. A recent study showed that a combination of these antibodies can predict development of IBD years before diagnosis. However, these serological markers appear to persist during the disease process, even after treatments.
For decades, AS has been viewed as a seronegative disease; distinctive for lacking autoantibodies which represent the hallmarks of autoimmune rheumative diseases such as rheumatoid arthritis and lupus. There has been a recent report of autoantibodies recognizing skeletal/connective tissue antigens in AS, but the relevance of these autoantibodies to AS pathogenesis remains unclear.
In WO 2013/010254, Applicant demonstrated that AS is characterized by elevated levels of autoantibodies (IgGs) against noggin (NOG) and sclerostin (SOST), but they exist mainly as immune complexes (ICs), implicating their role in structural damages. In sera, these ICs exist in a setting of antigen excess, thus accounting for previous difficulties in the identification of these autoantibodies. Elevated levels of NOG/SOST-IgG ICs are disease specific as normal levels of these ICs were found in individuals with mechanical back pain (MBP; the control group for ankylosis) and in IBD patients with no articular manifestations (the control group for inflammation).
NOG-SOST complexes with no antagonistic functions for both BMP and β-catenin signaling were identified from a rat osteosarcoma cell line, but their interacting sites have remained unknown. We have identified not only the sites whereby the two proteins bind, but also formation of ICs involving autoantibodies against NOG and SOST at their interacting sites (NOG-N54 and SOST-S146-specific IgG). This raised the intriguing possibility that these ICs may mimic the inhibitory interaction that naturally occurs between these two proteins and amplify the process. It is likely that the relative abundance of these epitope-specific ICs could fine-tune differential signaling of two different pathways influencing bone homeostasis (BMP and wnt/β-catenin). Our finding that AS patients have higher levels of these autoabs suggests that over-abundance of these autoantibodies would lead to reduced levels of functional NOG and SOST, resulting in enhanced BMP and β-catenin signaling. Neo-ossification would thus be promoted, ultimately leading to spinal ankylosis. The possibility that the NOG- and SOST-specific antibodies interfere with their antagonistic functions in bone formation suggests a specific link between autoimmunity and ankylosis in AS.